19 August 2011

App Note 23

"Micropower circuits for signal conditioning." 24 pages.

This app note discusses techniques for designing extremely low-power circuits.  As Jim says, "Although micropower ICs are available, the interconnection of these devices to form a functioning micropower circuit requires care."

The first two circuits are low-power temperature sensors (the first with a thermistor and the second with a thermocouple).  The next two circuits are strain-gauge amplifiers, which use sampling techniques to achieve micropower performance, exploiting low duty cycle to give low power consumption.  (Many more bridge circuits are coming up in App Note 43.)  The next three circuits are temperature-monitoring applications for the LTC1040, LTC1041, and LTC1042 family of micropower comparator circuits.  Figure 6 is a clever 4mA-to-20mA current-loop thermistor amplifier that uses the current loop signal as the power source.

Figure 9 is a micropower SAR analog-to-digital converter.  Successive approximation is a good technique for low-power A-to-D, but unfortunately, now that SA registers (like the 74C905 used here) have been discontinued, it's harder to implement in discrete form.  Figure 11 shows a micropower single-slope A-to-D converter, which only consumes 100 microamps (the recently discontinued 74C906 could easily be replaced by another low-offset CMOS switch).  Figure 14 is a micropower sample and hold (SAR A-to-D converters require a S&H front end). This circuit cleverly uses the programming pin on the LT1006 to turn down the power consumption in the hold mode.

The best circuit is Figure 16, a micropower 10-kHz voltage-to-frequency converter.  This circuit is also known as "The Zoo Circuit" and Jim wrote a chapter in his first book dedicated to it.  Note the quote on page AN23-13: "A nice day at the San Francisco Zoo…, instrumental in arriving at the final configuration, is happily acknowledged."  Rather than discuss the circuit here, I'll wait until I review his first book.  (However, I will note here that there is an error in the schematic: the base of Q4 should be connected to its collector.)  Figure 20 is a higher-speed V-to-F converter, reaching 1 MHz.

The final circuits are power regulators.  Figure 22 is a switching regulator (using a discontinued 74C907 as the switch, we've got the whole family now).  Figure 25 is a switching regulator that maintains a constant voltage drop across the LT1020 regulator, using the integral comparator.  I like the start-up circuitry here.  Figures 28 through 31 show off other tricks using the LT1020.

The box sections at the end (why not appendices?) cover a number of topics.  Box Section A discusses low-power techniques and the design evolution of the zoo circuit.  Box Section B discusses the LTC1040, LTC1041, and LTC1042 family of micropower comparator circuits.  And finally, Box Section C discusses the effects of test equipment on micropower circuits (or, stated another way, suggestions that might power your circuits from the input source: just turn up the amplitude on that pulse generator).

Best quote (from page AN23-19): For example, everyone "knows" that "MOS devices draw no current." Unfortunately, Mother Nature dictates that as frequency and signal swings go up, the capacitances associated with MOS devices begin to require more power. It is often a mistake to automatically associate low power operation with a process technology. While it's likely that CMOS will provide lower power operation for a given function than 12AX7s, a bipolar approach may be even better.

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